1. Field of the Invention
The present invention relates to an exhaust gas cleaning system having a particulate filter for collecting particulate matters included in exhaust gas of an internal combustion engine. Specifically, the present invention relates to an exhaust gas cleaning system capable of inhibiting deterioration of fuel consumption due to regeneration of a particulate filter.
2. Description of Related Art
Lately, reduction of particulate matters discharged from a diesel engine has been a major issue as an environmental measure. Therefore, an exhaust gas cleaning system having a diesel particulate filter (DPF) for collecting the particulate matters is disposed in an exhaust pipe. The DPF is generally made of porous ceramics having a multiplicity of exhaust gas passages. When the exhaust gas passes through porous filter walls providing the exhaust gas passages, the particulate matters are adsorbed and collected.
If the collected particulate matters keep on depositing in the DPF, a pressure loss may increase and engine efficiency may decrease. Therefore, the exhaust gas cleaning system has regenerating means for regenerating the DPF at suitable timing by combusting the collected particulate matters. As a method of the regeneration, an intake air quantity is decreased than usual, or a post-injection is performed after a main injection, for instance. Thus, the temperature of the exhaust gas introduced into the DPF is increased in order to combust and to eliminate the particulate matters in the DPF.
However, if the DPF is regenerated frequently by the above regenerating methods, fuel consumption is increased because energy is consumed to increase the temperature of the exhaust gas. Therefore, as an attempt to reduce the increase in the fuel consumption, a plurality of regenerating means having various energy efficiency and temperature-increasing performance is disposed, and an appropriate regenerating means is selected from them in accordance with an operating state of the engine. For instance, in an exhaust gas cleaning system proposed in Japanese Patent Unexamined Publication No. 2000-213332, a range of the operating state of the engine is divided into a plurality of ranges, and an optimum regenerating means is selected for each range. In the system, the most frequently used range of the operating state is determined based on a record of the operating state. Another range of the operating state is selected as a selection range so that regenerating means selected in the selection range has a lower fuel consumption ratio than that of the regenerating means of the most frequently used range. If the quantity of the particulate matters collected by the DPF exceeds a predetermined value and the operating state enters the above selection range, the regeneration is performed.
In the above system, a multiplicity of the regenerating means has to be provided, and the regenerating means has to be switched in accordance with the range of the operating state. Moreover, the most frequently used range varies by a vehicle driver. Therefore, the record of the operating state has to be detected for each vehicle driver, and the selection range has to be set for each vehicle driver. As a result, control of the regeneration is complicated.